Silver nitrate which is of very high purity has many important industrial applications, for example, it is required for the manufacture of photographic materials, for certain catalytic uses, and for use in the pharmaceutical industry. In the process that is generally used for preparing silver nitrate, metallic silver is dissolved in nitric acid and the contaminants associated with the metallic silver will contaminate the silver nitrate, unless appropriate steps are taken in the manufacturing process to effectuate their removal. A very wide variety of different contaminants, particularly metallic contaminants, can be present; with the specific contaminants involved and the amounts of each depending on the source of the metallic silver used as a starting material in the process. The contaminants include polyvalent metals such as copper, iron, lead, nickel, tin, bismuth, zinc, chromium, manganese, antimony, cadmium, gold, iridium, palladium, platinum, rhodium, mercury, calcium and magnesium; monovalent metals such as sodium, potassium and lithium; and anions such as sulfate, chloride, bromide, iodide, fluoride, sulfide, phosphate, nitrite, selenite, arsenate, borate and tellurite. Over the years, many different processes have been proposed to achieve removal of such contaminants, and thereby produce silver nitrate of very high purity.
Among the many patents describing the preparation and purification of silver nitrate, the following are representative:
(1) Marasco et al, U.S. Pat. No. 2,543,792, issued Mar. 6, 1951.
This patent describes a process in which an aqueous silver nitrate solution is brought into successive contact with elemental carbon, activated alumina and silver oxide to remove metallic impurities.
(2) Moede et al, U.S. Pat. No. 2,614,029, issued Oct. 14, 1952.
This patent describes a process in which an aqueous silver nitrate solution is treated with sufficient silver oxide to attain a pH of at least 6.1, precipitated metals and metal hydroxides are separated from the solution, and the solution is brought into contact with a water-insoluble, porous, solid adsorbent such as activated alumina or magnesia.
(3) Moede, U.S. Pat. No. 2,940,828, issued June 14, 1960.
This patent describes a process in which an aqueous silver nitrate solution is treated with sufficient silver oxide to attain a pH of at least 6.1, the solution is exposed to ultraviolet light, then filtered, and then brought into contact with a water-insoluble, porous, solid adsorbent such as activated alumina or magnesia.
(4) Dietz, U.S. Pat. No. 3,141,731, issued July 21, 1964.
This patent describes a two-step process in which an aqueous silver nitrate solution is first treated with silver oxide to increase the pH and precipitate certain of the contaminants, and then heated to a temperature of about 75.degree. C. to about 95.degree. C. and further treated by addition of iron, usually in the form of iron nitrate, and silver oxide to precipitate the remaining contaminants.
(5) Green, U.S. Pat. No. 3,554,883, issued Jan. 12, 1971.
This patent describes a process comprising the steps of mixing a silver nitrate solution with silver oxide in a proportion sufficient to give a pH in the range from about 5.1 to about 5.8 and form a precipitate, removing the precipitate to leave a partially purified solution, mixing the partially purified solution with silver oxide in a proportion sufficient to give a pH in the range from about 5.9 to about 6.3 and form a second precipitate, and removing the second precipitate to yield a purified solution.
(6) Long et al, U.S. Pat. No. 3,800,030, issued Mar. 26, 1974.
This patent describes a process in which gaseous acetylene or methylacetylene is bubbled into a silver nitrate solution to cause a selective reaction with the silver and the reaction product is separated to thereby leave the contaminants in solution.
(7) Asai et al, U.S. Pat. No. 4,136,157, issued Jan. 23, 1979.
This patent describes a process in which metallic silver is dissolved in nitric acid to form a silver nitrate solution, aluminum ion is added to the silver nitrate solution, the pH is adjusted by addition of silver oxide to thereby form a precipitate, and the precipitate is separated from the silver nitrate solution.
The processes of the prior art suffer from serious deficiencies which have hindered their industrial utilization. For example, some of these processes are complex and costly, and some involve steps which are quite hazardous. Moreover, they are often ineffective in providing silver nitrate of a very high degree of purity. Such silver nitrate is referred to herein as "ultra-pure" silver nitrate, by which is meant silver nitrate in which all, or almost all, contaminants have been reduced to exceedingly low levels, such as levels of below one part per million by weight and often below one tenth of one part per million by weight. Ultra-pure silver nitrate is particularly valuable in the manufacture of photographic materials in that many contaminants, even when present in exceedingly small amounts, can have very serious adverse effects on the properties of such materials. Thus, the present invention is of particular benefit to the photographic industry, although it has utility wherever silver nitrate of exceptional purity is needed.
It is toward the objective of providing an industrially feasible and economically practical process that will provide ultra-pure silver nitrate that the present invention is directed.